This invention relates to the art of electrostatographic copying, an electrostatographic photoreceptor and to a method of treating such a photoreceptor for use in electrostatographic copying machines. This form of copying, originally disclosed by C. F. Carlson in U.S. Pat. No. 2,297,691, involves as an initial step the uniform charging of a plate or drum comprised of a conductive substrate normally bearing on its surface a non-conductive barrier layer which is covered by a layer of a photoconductive insulating material. This is followed by exposing the plate or drum to activating radiation in imagewise configuration which results in dissipation of the electrostatic charge in the exposed areas while the non-exposed areas retain the charge in a pattern known as the latent image. The latent image is developed by contacting it with an electroscopic marking material commonly referred to as toner. This material is electrostatically attracted to the latent image which is, by definition, in the configuration of those portions of the photoreceptor which were not exposed to the activating radiation. The toner image may be subsequently transferred to paper and fused to it to form a permanent copy. Following this, the latent image is erased by discharging the drum and excess toner is cleaned from it to prepare the drum for the next cycle.
The photoconductive insulating material is characterized in that it has a comparatively high electrical resistance in the dark which resistance decreases significantly upon exposure to activating radiation. Both organic materials, such as 2,4,7-trinitro-9-fluorenone in poly(vinylcarbazole), and inorganic materials, such as amorphous selenium, have been successfully used as the photoconductive material in electrostatographic copiers.
Amorphous selenium has many desirable properties which render its use highly beneficial in electrostatographic copying machines. However, the advent of high speed copiers which make only one or two copies per drum revolution has necessitated the use of photoconductive materials which discharge at a faster rate than pure selenium. This is the case since the increase in copying speed is obtained by increasing the speed of drum revolution, hence lower exposure. Accordingly, the photoreceptor must go through the charge, expose, develop, transfer, discharge and clean cycle very rapidly. Researchers have discovered that the photodischarge rate for the same light intensity can be increased by combining the selenium with arsenic to form an alloy. In addition the use of selenium/arsenic alloys results in a photosensitive device which is sensitive to longer wavelengths of light than are those which employ pure selenium as the photoconductor. This concept is more fully disclosed by Ullrich in U.S. Pat. No. 2,803,542. Thus, alloys of selenium containing from about 0.3 percent up to about 48.7 weight percent arsenic are advantageously used as the photoconductive material in high speed electrostatographic copiers. The arsenic containing selenium alloys have been found to suffer from a drawback which has come to be known as print deletion. This phenomena is observed as blank areas in the copy which get progressively larger until the copy quality becomes unacceptable. It is not fully understood what causes print deletion, but it is believed that chemicals found in the ambient of some locations where copiers are placed contain constituents which react with arsenic in the photoconductor to form a conductive reaction product which allows lateral migration of the charge immediately after the charging step. As a result, no latent image is formed in these areas and print deletion results.
One solution to the aforementioned print deletion problem is disclosed in copending application Ser. No. 558,027 filed Mar. 13, 1975. This application discloses a particular polymeric material useful for overcoating electrostatographic photoreceptors. The overcoating material is disclosed as comprising a crosslinkable polymeric composition of:
i. a first polymer which is the addition polymerization product of methyl methacrylate, n-butylmethacrylate and acrylic or methacrylic acid, and
ii. a second polymer which is the addition polymerization product of styrene and maleic anhydride
This overcoating has proven itself highly effective when used in conjunction with selenium/arsenic photoconductors. Its use virtually eliminates the print deletion problem and has also been found to provide a significant improvement in copy quality. In addition, it has been observed that light fatigue problems associated with selenium/arsenic photoconductors are significantly reduced when the photoconductor is overcoated with this material.
Certain electrostatographic copiers employing selenium/arsenic photoconductors are cleaned by the application of a flexible doctor blade thereto and providing relative motion between the blade and the drum bearing the photoconductor on its surface. The doctor blade is typically made of a flexible material, e.g. polyurethane, in order for it to conform to the irregularities in the drum's surface. The use of a flexible blade necessitates the application of some lubricating material to the drum's surface to reduce the friction between the blade and the drum and thereby prevent blade chatter and foldover. It would simplify operation of the cleaning mechanism if the friction between the cleaning blade and the drum could be reduced to thereby avoid the need for the periodic application of a lubricating material to the drum's surface.
It would be desirable and it is an object of the present invention to provide a novel overcoating for electrostatographic photoreceptors.
An additional object is to provide such an overcoating which enhances the quality of copies produced by the overcoated photoreceptor.
Another object is to provide such an overcoating which reduces or eliminates the problems of copy deletion and light fatigue recovery associated with the use of selenium/arsenic alloys as photoconductors.
A further object is to provide such an overcoating which exhibits a significant reduction in friction between its surface and the surface of flexible doctor blades used to clean it.